Joe Biden lamented “too much violence, too much fear, too much grief” after the latest US mass shooting as he prepared to visit Uvalde, where police face intensifying scrutiny for waiting outside the classroom where a teenage gunman with an assault rifle killed 19 children and two teachers.

The US president and first lady, Jill Biden, arrived in Uvalde, Texas, on Sunday morning, both dressed in black. They visited the informal memorial of flowers and notes that has accumulated outside Robb elementary school, where the carnage took place last Tuesday.

Then they will meet families who lost loved ones, and those who survived the gunman’s rampage, followed by first responders, after a relatively long gap between the tragedy in Uvalde and the presidential visit.

On Saturday, the vice-president, Kamala Harris, called for a ban on such military-style assault weapons for the general public, while she attended the last funeral of the 10 people killed just two weeks ago in a racist attack on a supermarket in Buffalo, New York, carried out with a similar gun.

The center of Uvalde was busy but hushed on Saturday afternoon as a long line of people lined up quietly in heat approaching 100F, waiting to place flowers and other tributes at the hurriedly-created memorial of crosses set up for those killed five days ago at nearby Robb elementary school.

An ambulance was standing by and a state trooper assisted members of the public who came to mourn.

But as well as grief there was anger that has been simmering since Tuesday, when local police waited at least an hour, while young children trapped with the gunman repeatedly called 911 and parents outside pleaded with officers to go in, before federal agents arrived and shot the 18-year-old local man dead.

The police department specifically assigned to oversee school security in the area, led by Pedro Arredondo, appeared not to have followed state protocols advising that an “officer’s first priority is to move in and confront the attacker”.

The head of the Texas department of public safety, Steve McCraw, admitted on Friday afternoon that “of course it was the wrong decision” for local officers to wait to enter the classroom.

And Texas’s governor, Greg Abbott, said he felt “misled” and was livid after several days of conflicting accounts about the law enforcement response.

Biden spoke about the tragedy in Uvalde during a commencement speech he gave on Saturday morning at the University of Delaware, his alma mater.

“As I speak, those parents are literally preparing to bury their children. In the United States of America. Too much violence, too much fear, too much grief,” said Biden and called the Uvalde and Buffalo mass shootings acts of “evil”.

“In the face of such destructive forces, we have to stand stronger. We cannot outlaw tragedy, I know, but we can make America safer. We can finally do what we have to do to protect the lives of our people, and of our children,” he said.

The US has received increasing criticism from the international community and gun safety advocates domestically over continual mass shootings and the failure of lawmakers to pass gun control laws that could mitigate them.

Biden and fellow Democrats have been repeatedly out-maneuvered in the last decade by Senate Republicans, many of whom are backed by the powerful gun lobby.

Harris called on the Congress to act, saying: “We are not sitting around, waiting to figure out what the solution looks like. We know what works on this. It includes – let’s have an assault weapons ban.”

In Uvalde, Alfred Garza was among several parents who gathered outside the elementary school after reports that a shooting was under way and witnessed officers delaying a move to storm in. He tried not to get in the way. Other parents begged officers to take action.

His daughter Amerie Jo, 10, was among those shot dead before federal agents arrived and killed the gunman, Salvador Ramos.

“It doesn’t take a genius to figure out that it just took too long to get in there and, you know, had they gotten there sooner, and someone would have taken immediate action, we might have more of those children here today, including my daughter,” he told CNN.

Warning signs about Ramos had been evident prior to his attack, with reports of threatening posts on social media and aggressive interactions with teenage peers.

But he was able legally to arm himself with assault rifles and hundreds of rounds of ammunition shortly after he turned 18 this year.

His mother, Adriana Martinez, gave a short television interview earlier in the week, saying in Spanish: “I have no words to say. I don’t know what he was thinking,” adding: “He had his reasons for doing what he did. Please don’t judge him. I only want the innocent children who died to forgive me.”